This is kind of a two part question. Do you mean the mom measures small, or the baby measures smaller and ends up bigger?

I measured right along where I was supposed to be with DS. They estimated him at 7 - 7 1/2 lbs and he was born 10 days early at 8 lbs 15 oz. Those estimates can vary a lb in each direction on average though. But they didn't expect him to be that big at all.